1. Technical Field
This invention relates generally to frequency correction in wireless communication devices, and more particularly to a method and apparatus utilizing a single system clock for communicating with multiple radio access technologies.
2. Background Art
Mobile communication devices, such as mobile telephones, communicate with towers and base stations by synchronizing internal circuitry with a communication signal being delivered by a serving cell, tower, or base station. These devices perform the synchronization by adjusting or correcting the frequency of an internal oscillator so as to align the internal oscillator with the frequency of a carrier signal associated with the serving equipment. For example, a mobile telephone that is in communication with a cell tower will frequency correct its internal oscillator so as to be frequency and time synchronized with the tower carrier signal while a call is in progress.
In a traditional mobile telephone, synchronization of a frequency synthesizer is performed to allow proper modulation or demodulation of the communication signal and synchronization of a system clock is performed to maintain proper time alignment with the serving equipment. The frequency synchronization is often performed by a pair of phase locked loops working as frequency synthesizers. A first phase locked loop is synchronized with the base station carrier signal for proper modulation or demodulation of the communication signal, while a second phase locked loop frequency synchronizes a system clock which is used to keep track of system time critical events. Such a system works well for mobile telephones configured to communicate with a single radio access technology, such as TDMA, CDMA, GSM, WLAN, or WiMAX.
In today's mobile society, people often travel from state to state and country to country. When traveling from one region to another, people sometimes travel into areas being served by radio access technology networks different from the radio access technology they typically use. For instance, a person with a CDMA phone may travel into an area being served by a GSM network. As the carrier and modulation frequencies associated with CDMA and GSM technologies are different, a conventional CDMA mobile phone will not be capable of communicating with the GSM network. Consequently, the need for a mobile communication device capable of communicating with multiple technologies has arisen.
The problem with this need is that to be able to communicate with multiple radio access technologies, a mobile device must have circuitry capable of communicating with multiple carrier signal types, each of which may have different carrier frequencies and different modulation systems. One proposed solution to this problem is to equip the mobile device with redundant communication circuitry. A multimode device may employ multiple system clocks, multiple frequency correction devices for the system clocks, multiple modulators and demodulators, and multiple frequency correction devices for the modulators and demodulators. While such a system works in practice, it is expensive to manufacture in that duplicate components must be added to each device. Further, the duplication of components increases the overall size of the device. Additionally, the mean time between failures of these devices is reduced, as a higher component count offers more opportunities for any one part to malfunction.
There is thus a need for an improved apparatus and method capable of communicating with multiple radio access technologies without the need of duplicative communication circuits.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.